New Pistons..

[Astrid W108 280SE]

Hi Everyone,

I have just purchased six new pistons for my M130 (1970 280SE) engine and the ones that I have bought have a beveled edge to them which look a wee bit different to the existing pistons that are flat. Can anyone see a real issue with this?

These are an extra millimetre larger than the last ones - are there any special things the person that is rebuilding the engine should know that are perculiar to Mercedes Benzes. They seem quite competent - but want to make sure there are no slip ups.

Cheers

Piston.jpg

D

[Mercmad]

Those arer low compression pistons . he step is designed to lower the compression to compensate for the overbore, so that tuning remaines the same. If you bore out an engine,the compression ratio changes,thus Daimler factored this into the design.

[Astrid W108 280SE]

Thanks Mercmad! Those clever Germans think of everything.

Apart from re-boring the extra 1mm - is there any thing else perculiar that we should be mindful of? Piston set didn't come with any information in regards to clearances (not sure if my terminology is right there) for the rebore which my mechanic was surprised about. Just want to get this baby perfect.

The ring set also came with four rings - whilst the piston only has three, and I have been told by the supplier that the first, third an forth should be used as the second is only used on piston with four rings (ring set is compatible with different types of pistons apparently). Does this sound right?

Cheers

D

[cuisses]

Those arer low compression pistons . he step is designed to lower the compression to compensate for the overbore, so that tuning remaines the same. If you bore out an engine,the compression ratio changes,thus Daimler factored this into the design.

That is very clever. When I first read this my initial reaction was "Bullshit - the compression ratio just depends on the stroke and the gap at the top of the cylinder at top dead centre, and they don't change when you overbore." However, I was wrong. The combustion chamber is not just a cylinder, but a cylinder plus the bit in the head - and the volume of the bit in the head is constant (at least approximately) when you rebore so it turns out as you bore more you get slightly higher compression. A good question for the students.

[kimrh]

Interesting!!
I have new sets of M117-968 10.1 high comp pistons in both +1 and +2
Standard original factory piston being 96.500mm / +1 being 96.505mm / +2 being 96.510mm
+1 for simple light tidy-up of the bores - followed up etching/lapping with special felt pads (allusil bores) - so 0.005mm of material removed
+2 for removing scouring of the bores -followed up with etching/lapping with special felt pads - so 0.010mm of material removed
Both sets are identical crowns as the original 96.500 pistons - so that would bump compression up slightly in this case

[Tony From West Oz]

The difference in compression ratios, by adding .005mm to the diameter of a 95mm diameter piston, would not be significant.
Even an extra 1mm in diameter would not make a significant difference to the compression ratio.
The area of the piston (neglecting the additional protrusion of the rings - which would be constant regardless of the oversize) would be:

(95mm Diameter/2) squared * Pi (3.14) = 7088.2 sq mm
(96mm Diameter/2) squared * Pi (3.14) = 7238.322947383 sq mm
7238/7088 = 1.0211 or 2% larger in piston area.


Should you see any error in my calculations, please post the corrections.
Regards,
Tony

[cuisses]

The difference in compression ratios, by adding .005mm to the diameter of a 95mm diameter piston, would not be significant.
Even an extra 1mm in diameter would not make a significant difference to the compression ratio.
The area of the piston (neglecting the additional protrusion of the rings - which would be constant regardless of the oversize) would be:

(95mm Diameter/2) squared * Pi (3.14) = 7088.2 sq mm
(96mm Diameter/2) squared * Pi (3.14) = 7238.322947383 sq mm
7238/7088 = 1.0211 or 2% larger in piston area.


Should you see any error in my calculations, please post the corrections.
Regards,
Tony

No Tony, this is not what you do.



Compression ratio= Volume at bottom dead centre/Volume at top dead centre

compratio1.jpg

If the combustion chamber was just a pure cylinder with a flat top (like the simple figure above) then CR would not change at all when you bored it out. Suppose the cylinder stopped a distance d from the top when at tdc, and it was at a distance h from the top when it was at bdc. h and d are set by the crank and conrod and piston rod and piston height and that doesn't change when you rebore.

The compression ratio is then

CR = (pi r^2 h)/(pi r^2 d) = h/d

The r's cancel so boring has no effect. You can choose whatever r you like and you still get the same compression ratio. However, this calculation is rubbish, because the combustion chamber is not a pure cylinder but a cylinder plus the strange bit in the head, and this complicates things. It looks a bit like the figure below , and there are all these extra volumes don't change with r^2, and so the r^2's no longer cancel.

engine_compression_ratio.jpg

There is a compression ratio calculator here which you can play around with. It uses inches and cc's together! :

https://www.rbracing-rsr.com/compstaticcalc.html

I do agree with you that a rebore is not likely to make much difference in the compression ratio, but these Mercedes engineers are not going to be happy with "close enough".

[CraigB]

I wouldn't want to upset anyone by claiming that pistons aren't round and that you have to add that to your formulas David! My understanding is that pistons are ever so slightly oval, wider on the thrust faces. If you are building a race motor you increase that ovality so you have less piston contact and friction, but of course greater wear. My cousin has a piston grinding machine where you can dial up the different grinds. But of course talking even smaller proportions. If you look at how much is missing off that piston, would also be easy to lose that out of the head chamber I guess. Still, gotta love that attention to detail.

Back to the clearance question, I recall my cousin boring my Amilcar (1924 and a lot older of course) but he was saying they would often get engines to re - rebuild at AES where shops had rebored to 'modern tolerances' that are way too tight for the old alloys etc that are less controlled in expansion etc. Does anyone like on have actually stats from Benz literature as to what to rebore to? If not, i can call my cousin or the old shop foreman Ewald and ask them for some guidance. But my understanding is that the cold clearances on modern motors are way tighter than what they used to be, and that helps with emmisions etc, and that might even be ok if you have new material pistons, but the risk is that once hot the whole show could seize up, particularly when all fresh and new, if there is not enough clearance. Sounds like your mechanic is switched on enough to know that and hence his question.

[cuisses]

I wouldn't want to upset anyone by claiming that pistons aren't round and that you have to add that to your formulas David! My understanding is that pistons are ever so slightly oval,

Yes, you are right, but they wear to be oval due to the fact that the thrust from the piston rod is in one plane only. I don't think the fact that they are non-circular is a big effect. Also, when you are reboring a cylinder it would be pretty difficult to make it an oval, although maybe the experts do. I wasn't trying to suggest I knew what I was doing, more that I had he idea that it was OK to approximate the combustion chamber as a circular cylinder, but for some circumstances this is wrong.

[CraigB]

I'm never good at explaining these things, but my understanding is new engine, bores are perfectly cylindrical. Pistons are oval, out of the box if you measure the new ones they will be ever so slightly oval. If you wanted to make your own pistons, you wouldn't just put them in a lathe and turn them, you would need to finish them off in a piston grinding machine that give them that ovality. I seem to recall on my cousins machine it had something like 3 or so different grinds. For maximum longevity it is closer to round with lots of surface area to take the thrust as the power pushes it down and against the cylinder wall in line with the rotation of the crank. But if you were building a race motor to just last a season or so, then by being much more oval there is much less contact area and drag, but of course the rings are still sealing against the round bore, but you don't need it to last many miles before you rebuild and check them. If you see some of the modern real high performance pistons, they have hardly any contact area and more than just oval, they are missing great areas at the side.

They will as you say wear, but i think you check that by measuring top and bottom of piston for its diameter across that thrust plane.

But i am an untrained amateur - a reteller of stories from a master, but we all know what can happen when someone retells something they don't understand...... and i am too lazy to google and try and look that up. And of course my cousin must be something like 65 now, so he can tell you how to squeeze every last bit of power out of a flat head V8 or Austin 7 etc, but not so much the modern stuff - but has done a lot of Mercedes stuff over the years.

[Astrid W108 280SE]

Thanks everyone for the very thorough replies - am feeling more and more like a numpty with each complex formula that is added.

It is ALL much appreciated!!

Cheers

D

[Astrid W108 280SE]

Well I did a bit of digging on the interwebs and found this wee handy bit of information.....it might come in handy for someone down the line...

Cheers

D